The present invention relates to an image processing apparatus for color image formation apparatus, an image processing method and an image processing program, and more particularly, to selection of color separation table for separating a color image signal into color-material (ink or toner) colors used in a color image formation apparatus.
In a color printer or the like, various colors are reproduced by using a limited number (e.g. 4 or 7) of color materials. For example, in an ink-jet color printer using CMYK inks, various neutral colors can be represented on a print medium by appropriately controlling C-ink discharge amount, M-ink discharge amount, Y-ink discharge amount and K-ink discharge amount.
In this manner, a large number of colors can be represented by controlling the respective YMCK ink discharge amounts, however, the respective ink-discharge amounts are different for representation of respective colors. For this reason, the total ink amount differs depending on represented color, grayscale level and the like. The total ink amount means the total sum of the respective ink discharge amounts necessary for representation of a color.
Some color can be represented with a small total ink amount, but some can be represented with a large total ink amount. This means a color requiring a minimum total ink amount and a color requiring a maximum total ink amount are included in colors in CMYK color space. Especially, upon ink discharge when the total ink amount is the maximum value, attention must be paid to ink overflow. As an absorbable ink amount (allowable discharge amount) of print medium is physically determined, the discharge amounts must be controlled such that the total ink amount does not exceed the absorbable ink amount.
FIG. 10 shows an example of an image processing apparatus of a color printer related to the present invention. In FIG. 10, reference numeral 101 denotes a color matching processor; 102, a color separation processor; 103, an output γ correction processor; 104; a halftone processor; 105, a color printer engine; 1006, a color separation table unit; 1007, an output γ correction table unit; 1008, maximum total ink amounts; and 1009, print media.
The color matching processor 101 performs color matching processing for matching between color reproduction characteristics of input image data RGB and printer colors. The color separation processor 102 performs color separation processing by interpolation processing such as tetrahedral interpolation, from input multilevel R′G′B′ image data to color-printer material colors (hereinbelow ink colors), cyan (C), magenta (M), yellow (Y) and black (K), based on table information of the color separation table unit 1006. The output γ correction processor 103 corrects gray scale characteristics of the multilevel CMYK data outputted from the color separation processor 102 via a 1-dimensional LUT (Look-up table) corresponding to the color reproduction characteristics of the color printer engine 105, and outputs multilevel C′M′Y′K′ data. This processing realizes correction processing corresponding to the characteristics of the color printer.
Note that in the above-described image processing, a maximum value of the total CMYK ink discharge amount is defined by the combination of the color separation table unit 1006 and the output γ correction table unit 1007. The halftone processor 104 performs halftone processing for the printable number of grayscale levels, e.g., binarization if the printer is a binary printer, on the corrected multilevel C′M′Y′K′ data, and outputs C″M″Y″K″ binary data. The color printer engine 105 performs printing based on the input C″M″Y″K″ binary data. The maximum total ink amounts 1008 indicate the total ink-amount maximum values allowable for the print media A to C 1009.
Especially in the related art of the present invention, optimum color separation tables and corresponding output γ correction tables are independently prepared for respective existing print media. For example, as tables for print processing for the print medium A, a color separation table A and an output γ correction table A are prepared. Further, as tables for the print processing for the print medium B, a color separation table B and an output γ correction table B are prepared. Further, as tables for the print processing for the print medium C, a color separation table C and an output γ correction table C are prepared. Further, as a maximum total CMYK ink-amount allowable for the print medium A is 200%, the maximum total CMYK ink amount defined by the combination of the color separation table A and an output γ correction table A is 200%. Similarly, regarding the print media B and C, the maximum total CMYK ink amounts are respectively 50% and 300%.
In a case where an unknown print medium which has not been expected as a medium to be used is employed, as a specified γ correction table does not exist, γ correction of CMYK colors is not appropriate, and the output γ characteristics are unbalanced.
Further, regarding the color separation table, as an appropriate color separation table does not exist, if a color separation table for an inappropriately large maximum total ink amount is used for the unknown print medium, the amount exceeds the ink absorbable amount and the ink overflows on the print medium.
On the other hand, if a color separation table for an inappropriately small maxim total ink amount is used, the range of color reproducibility is small although the range can be enlarged by using larger amounts of inks.
Further, in a case where a color separation table appropriate to an unexpected new print medium, as complicated processes are required, much time is required for generation of the color separation table, and the table cannot be easily generated.